Producing petroleum from underground formations



XR anti 39574 nited States PRODUCING PETROLEUM FROM UNDER- GROUND FORMATIONS Joseph C. Allen, Bellaire, Tex., assignor to The Texas Company, New York, N.Y., a corporation of Delaware No Drawing. Application December 14, 1956 Serial No. 628,235

14 Claims. (Cl. 166-2) force useful in the displacement or recovery of petroleum from underground petroleum-containing formations is the so-called solution gas drive wherein gas dissolved in the liquid petroleum present in theunderground formation serves as the driving force to expel or displace the liquid petroleum therefrom. In-a solution gas drive the gas released from the liquid petroleum upon pressure reduction serves to displace and move the petroleum from the formation in the direction of the zone therein of reducing pressure.

In some instances, however, solution gas drive is ineificient. Frequently the main cause for the inefficiency of a solution gas drive in the displacement of liquid petroleum producing formation is due to the tightness or atent O low permeability of the petroleum producing formation.

itself. Low permeability within the petroleum producing formation, particularly in the area of the well bore, usually necessitates rather high drawdown pressures around the well bore in order to maintain a given high rate of production. At these relatively high drawdown pressures free gas is formed in the zone surrounding the well bore within the producing formation. This free gas usually leads to the development of gas permeability or gas fingering around the well bore which permits excess gas to flow. Excess gas is the free gas (natural gas) that is produced and numerically is equal to the total gas produced minus the solution gas still remaining within the liquid produced petroleum. Production of excess gas 7 ,means that some of the petroleum left in the formation is denuded or has lost some of its original solution gas.

.Accordingly, it is an object of this invention to provide an improved method for the production of petroleum from an underground petroleum-containing formation.

Still another object of this invention is to provide a method of conserving so-called solution gas energy within a petroleum-producingformation during the production of liquid petroleum therefrom.

Still another object of this invention is to provide a method for increasing the production of liquid petroleum from a petroleum-producing formation.

Yet another object of this invention is to provide a method for maintaining reservoir pressure within a mum leum producing. formation.

Still another object of this invention is to provide an improved method of secondary recovery wherein a displacing fluid is introduced into a petroleum-containing formation to displace the petroleum therefrom.

How these and other objects of this invention are accomplished will become apparent with reference to the accompanying disclosure. In at least one embodiment of the practice of this invention at least one of the foregoing objects will be achieved.

in accordance with the practice of this invention production of petroleum from an underground petroleum containing formation is enhanced by introducing into said formation a naphtha fraction having dissolved therein a substantial amount of a normally gaseous hydrocarbon or a mixture of normally gaseous hydrocarbons, such as natural gas. The normally gaseous hydrocarbon is dissolved in the light petroleum fraction at a relatively elevated pressure, for example, at a pressure of at least 500 p.s.i.g., Le, a pressure at least sufficient to substantially increase the volume, e.g., by at least about 10%, of the original light petroleum fraction prior to having the normally gaseous hydrocarbon dissolved therein.

It has been determined that there exists a difference in the solubility of a normally gaseous hydrocarbon, such as methane, ethane and propane and'the like or mixtures thereof, such as natural gas, in a total crude oil as compared with a light hydrocarbon fraction, such as a naphtha fraction thereof. It has been further determined that a light petroleum fraction, such as a naphtha fraction having a boiling range in the range 425 R, will dissolve or absorb more of a given hydrocarbon gas under given conditions of temperature and pressure than a total crude. Further, it has also been observed that under a given'set of conditions of temperature and pressure a normally gaseous hydrocarbon, such as methane or ethane or mixtures thereof, such as are to be found in natural gas, provide a solution of greater volume when dissolved in a given volume of light petroleum fraction, such as a naphtha fraction, than when dissolved in the same volume of a heavy petroleum fraction or a total crude oil which itself may be the source of the aforesaid light petroleum fraction. This phenomenon has been observed in the case of methane and ethane by variations in so-called apparent density.

Apparent density is a function of not only the density of the liquid in which these gases are dissolved but also of the quantity of the gas that is dissolved in the liquid. The lighter the liquid the lower is the apparent density of the dissolved gas. Also, as the quantity of the dissolved gas in a given amount of liquid increases the socalled apparent density of the dissolved gas decreases. Thus it has been determined that a light petroleum fraction, such as a naphtha fraction, e.g., a light straight run naphtha having a boiling range in the range 90250 F., -would be an excellent vehicle for the return of the produced natural gas or equivalent normally gaseous hydrocarbon to the petroleum reservoir for the purpose of maintaining reservoir pressures and/or increasing the recovery of liquid petroleum therefrom. For example, when the produced gas is returned to the reservoir dissolved in a light petroleum fraction the returned gas would occupy a larger volume in the reservoir than it did originally when dissolved in the total crude or petrov means-m Original Oil Gasoline Fraction Oil Gravity Reservoir Temperature. Bubble Point Pressure 134 F 0.292 bbl. gasoline X950 (G/O) =27? ft. of produced gas dissolved by the gasoline traction at 1,600 p.s.i.a. and 75 F. or at 1,850 p.s.i.a. at i34 F of produced gas returned to reservoir in solution in gasoline. 0.292 bhl. gas saturated gasoline X1.57 (Formation Volume Factor) =0.46 bbl. at reservoir condition returned to reservoir per 1.345 bbls. reservoir volume voided. Ratio of volume of dissolved gas to volume of gas at Standard conditions:

Gas in Crude Oil 1.345-l.000=0.345X5.6+655=0.0295 vdhmmd/v... Gas in Gasoline- ]..57-1.00=0.57X5.6+950=0.0335 Vdlllol'ld/VI-O- As the data in accompanying Table I indicates, at a bubble point pressure of 1850 p.s.i.a. and a reservoir temperature of 134 F. the original oil contained 655 cubic feet of gas in solution per barrel of stock tank oil.

ture Correlation for Mixtures of California Oils and Gases" by M. B. Standing, page 275, AH Drilling and Production Practice (1947). This method was used in order to calculate the gas-oil ratio and formation volume of'the gasoline fraction from the crude oil. The close agreement between the calculated and the experimental properties for the crude oil indicates the validity of the correlation. It is noted that the gasoline fraction which comprised only about 30% by volume of the oil (stock tank oil) is capable of dissolving 950 cubic feet of gas per barrel gasoline as compared to 655 for the oil itself. It is apparent, therefore, that the formation volume factor is much greater for the light petroleum fraction (gasoline) than the oil itself. This 30% gasoline fraction itself will dissolve 42% of the produced gas. Upon returning the saturated gasoline to the reservoir 0.46 barrel of reservoir space would be replaced for each 1.345 bar-' rels of reservoir space voided. Desirably all of the produced gas is returned to the formation. The 277 cu. ft. of dissolved gas and the approximately 30% gasoline fraction would occupy 0.46 barrel. An additional 0.616 barrel of injection fluid would be obtained from the remaining 378 cu. ft. of produced gas by obtaining the required additional amount of gasoline from an extraneous source fora total of 1.08 barrels of injection fluid. This ,7 would mean an 80% volumetric efliciency in the replacement of the produced reservoir fluid. Thus, in accordance with the practice of this invention a good pressure maintenance mechanism within the formation is obtained by introducing into the formation a light naphtha fraction (gasoline) saturated with a gaseous hydrocarbon, e.g., methane or the produced natural gas.

--The final calculation under Table I indicates that the gas in solution in gasoline within the reservoir would occupy about 14% more volume than the same quantity of gas when dissolved in the original formation oil. As Table I indicates, the light petroleum fraction or gasoline employed in the indicated practice of this invention might have a formation volume factor of about 1.6 when saturated at the indicated reservoir conditions with the produced natural gas (950 cubic feet per barrel). The ap- The r experimental bubble pointpressure was 1739 p.s.i.a and the 1850 p.s.i.a bubble point pressure was calculated according to a method entitled Pressure-Volume-Temperaproximate'API gravity of this saturated gasoline at reservoir conditions would be about 89. This gravity and density is very close to that of pentane.

A light petroleum fraction, such as a naphtha fraction or a gasoline fraction, is particularly suitable as a carrier fluid for returning the produced normally gaseous hydrocarbons (natural gas) back to the formation in accord ance with this invention. The returned fluid will carry methane in solution which would then tend to diffuse or mix with the virgin reservoir oil more efficiently than another type of fluid, such as liquefied petroleum gas (L.P.G.), since the lower the molecular weight or the smaller the molecule the greater its diffusion rate. This diffusion of the gas in solution in the light petroleum fraction would cause an increased mixing of the virgin reservoir oil with the injected gas-saturated solution.

Another advantage according to the practice of this invention wherein a light naphtha fraction substantially saturated at an elevated pressure with natural gas is injected into the producing formation is that the solution so injected would be at this bubble point pressure under reservoir conditions whereas an apparently similar type of fluid, such as L.P.G., on the other hand, when mixed with the reservoir oil would reduce the bubble point pressure of the resulting mixture. This would not occur when a solution in accordance with the practice of this invention is employed since the injected solution would be at the same bubble point pressure as the reservoir fluid.

As previously indicated, any suitable normally gaseous hydrocarbon, such as methane, ethane, propane or mixtures thereof, such as natural gas, may be employed in the practice of this invention. A natural gas suitable for use in the practice of this invention might have a composition in the range -99% by vol. methane and 9. 40%

1 by vol. ethane.

A petroleum fraction-particularly suitable for use in the practice of this invention is a straight run naphtha, such as a whole straight run naphtha having a boiling range in the range 90-425 F., more or less, or a particular fraction thereof, such as a light straight run fraction having a boiling range in the range l00250 F., more or less. As previously indicated, a low molecular weight normally liquid petroleum fraction is preferred. Accordingly, a light straight run fraction is preferred over a heavy straight run fraction.

In accordance with one embodiment of the practice of this invention improved production of petroleum from an underground petroleum-producing formation is possible by the injection of the saturated gasoline fraction directly back into the producing formation into another.

' portion thereof removed from that portion of the formation, such as around the well bore, wherein the petroleum is produced from the formation. Accordingly, reintroduction of tbesaturated gasoline fraction serves not only to maintain formation pressure but also to enhance the ultimate recovery of petroleum from the formation.

In accordance with another embodiment of the practice of this invention the saturated gasoline fraction is introduced into the formation via an injection well, the petroleum being produced from another portion of the formation via a producing well. As previously indicated,

7 the light petroleum fraction or gasoline saturated with results are obtainable since the solution thus-injected would be miscible with the petroleum in place and would be an effective displacing fluid because of the more favorable viscosity ratio existing between the injected fluid and the in place petroleum, particularly as compared to BMW an injected L.P.G. liuid' or the injection of natural gas alone. For example, whenever gas itself is injected into -a reservoir. the viscosity difierence between the in place petroleum .to be displaced and the injected gas is great and this contrast leads to fingering or channeling of the injected gas within the formation to the producing .well, thereby rendering the gas highly ineflicient in displacing the liquid petroleum toward the producing well. However, when this gas is in solution in a light petroleum fraction in accordancewith the practice of this invention a much more efiicient displacement of the in place petroleum results.

Further, in a secondary recovery type of operation involving an injection well and a producing well after breakthrough of the injected gasoline into the producing well the rate of production of the injected gasoline would increase. By this time, however, the formation would probably already have been producing excess gas. Thus, this extra amount of produced gasoline would be available at a most opportune time for dissolving and returning to the reservoir the large amount of produced excess gas. It is believed that this type of a secondary recovery operation, cycling of the formation by returning naphtha plus gas dissolved therein either for pressure maintenance or for displacing the petroleum from the formation, would be superior to the straightforward L.P.G. injection since it would carry methane in solution which would tend to diffuse the mix with the petroleum previously in place in the reservoir more efficiently than L.P.G. would. Eventually after having displaced and/or produced substantially all of the displace- ,/able in place petroleum from the formation production can be completed in the reservoir by pressure reduction or blowdown. Another suitable method of secondary recovery utilizing at least in part the teachings of this invention would be to follow the injected naphtha with a natural gas drive and/or a water flood.

Another procedure, still in accordance with the practice of this invention involving an injection Well and a production well and wherein produced naphtha saturated with gas is cycled through the formation, after production of substantially all of the petroleum from the reservoir, would be to process the produced gas by extracting the L.P.G. and natural gasoline therefrom. The L.P.G. and/or natural gas could then be returned to the formation to displace the previously injected light petroleum fractionor gasoline therefrom. Finally, the thus-introduced L.P.G. could be produced from the formation by pressure reduction or blowdown thereby resulting in substantially complete reduction of all the normally liquid hydrocarbons from the formation, leaving substantially only gas in place and this at a relatively low pressure.

As previously indicated, a preferred light petroleum fraction is the naphtha fraction recovered from the produced petroleum and returned to the producing formation after having been saturated with a normally gaseous hydrocarbon or mixtures thereof at a relatively high pressure. Desirably thelight petroleum fraction is saturated with a normally gaseous hydrocarbon at ambient surface temperature, such as a temperature in the range 50-250 F., at an elevated pressure, preferably at a pressure of at least 500 p.s.i.g., desirably at a pressure greater than 1,000 p.s.i.g., such asa pressure in the range 1000- 3000 p.s.i.g.

As will be apparent to those skilled in the art many changes and alterations may be made without departing from the spirit or scope of this invention.

I claim:

1. A method of producing petroleum from an underground petroleum-containing formation which comprises withdrawing petroleum from one portion of said formation and introducing into another portion of said formation a liquid petroleum naphtha fraction substantially saturated at an elevated pressure in excess of 500 p.s.i.g.

and at a temperature in the range 50-250' F. with-i1 normally gaseous hydrocarbon.

2. A method of producing petroleum from a subsurface petroleum-producing formation which comprises Withdrawing petroleum from said formation, separating a liquid naphtha fraction having a boiling point in the range -420 F. from said petroleum and reintroducing into said formation said liquid naphtha fraction sub stantially saturated with natural gas at an elevated pressure in excess of 500 p.s.i.g. and at a temperature in the range 50-250 F.

3. A method in accordance with claim 2 wherein said naphtha fraction is saturated with natural gas at an elevated pressure in the range 500-2500 p.s.i.g.

4. A method in accordance with claim 3 wherein said natural gas is comprised substantially only of methane.

5. A method of producing petroleum from a subsurface petroleum-containing formation which comprises withdrawing petroleum from said formation at a producing well, fractionating the withdrawn petroleum to separate therefrom the normally gaseous hydrocarbons and a liquid naphtha fraction, substantially saturating said separate naphtha fraction at an elevated pressure in excess of 500 p.s.i.g. and at a temperature in the range 50-250 F. with a normally gaseous hydrocarbon and reintroducing into said formation via an injection well the resulting liquid naphtha fraction.

6. A method in accordance with claim 5 wherein said naphtha fraction is saturated with natural gas at an elevated pressure in the range 500-250 p.s.i.g.

7. A method in accordance with claim 5 wherein said normally gaseous hydrocarbon is methane.

8. A method in accordance with claim 5 wherein said normally gaseous hydrocarbon is a gaseous mixture con taining methane and ethane.

9. A method in accordance with claim 5 wherein said separated normally gaseous hydrocarbons are employed to saturate said separated liquid naphtha fraction.

10. In the method of producing petroleum from an underground petroleum-producing formation wherein a displacing liquid is injected into said formation and employed to displace said petroleum from said formation the improvement which comprises employing as said displacing liquid a liquid petroleum naphtha fraction substantially saturated with a normally gaseous hydrocarbon at an elevated pressure in excess of 500 p.s.i.g. and at a temperature in the range 50-250 F.

11. In the method of producing petroleum from an underground petroleum-producing formation wherein said petroleum is produced from said formation by displacing said petroleum therefrom with a liquid introduced into said formation the improvement which comprises introducing into said formation a liquid naphtha fraction previously separated from the petroleum recovered from said petroleum-producing formation, said liquid naphtha fraction being substantially saturated with natural gas at an elevated pressure in the range 500-2500 p.s.i.g. and at a temperature in the range 50-250 F.

12. A method in accordance with claim 11 wherein said liquid naphtha fraction has a boiling point range in the range 90-425" F.

13. A method of producing petroleum from an underground petroleum-containing formation which comprises producing petroleum from said formation via a production well penetrating the same, separating natural gas and a liquid naphtha fraction from the produced petroleum, saturating said liquid naphtha fraction with natural gas at an elevated pressure in excess of 500 p.s.i.g. and at a temperature in the range 50-250 F. to substantially increase the volume of said naphtha fraction by at least about 10%, introducing the resulting liquid naphtha fraction into another portion of said formation via an injection well penetrating the same, carrying out the aforesaid operations until substantially all of the petroleum within said formation has been produced, then intro r. w e

7 8 ducing into said formation via said injection well a I References Cited in the file ofthis patent liquefied normally gaseous hydrocarbon to displace the UNITED STATES PATENTQ other hydrocarbons within said formation toward said y production well, producing the resulting displaced hydro- 699,832 Allen 1955 Allen May'17,1955

FOREIGN PATENTS carbons via said production well and subsequently pro- 5 21708381 ducing from said formation the introduced liquefied normally gaseous hydrocarbons. I

14. A method in accordance withclairn 13 wherein 696524 Great Bmam Sept 1953 'said liquefied normally gaseous hydrocarbons are pro- OTHER REFERENCES v duced y dlsPlacing Same with a gaseous displacing 10 Wharton et al.: Petroleum Engineer, April 1950, pp.

ageflt- 3-54, 3-58 to 3-60, B6 to 13-66, and 13-70. 

